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For much of its 50 million year history, the English Channel—or La Manche if you are looking from the French side—would have looked less like the ocean and more like a shallow valley with criss-crossing river canyons that did not completely sever Britain from the main European continent. Recently, scientists have postulated a megaflood is what has ended up separating the two landmasses on a permanent basis. Once a controversial theory, the idea of the “megaflood” of the English Channel is gaining more support with strong evidence from recent research that reveals additional physical traces of this unbelievable event.

The English Channel and the Strait of Dover (Image via NASA, Public Domain)

In 2007, Sanjeev Gupta and colleagues from Imperial College London tested a hypothesis that had been floated but never before examined in depth—that the English Channel was a site of megafloods and the floor of the valley contains erosional grooves that indicate it was once exposed to the air and eroded by an immense quantity of water. Bathymetry data of the channel floor show morphologically distinct cuts into the bedrock that would seemingly could have only been caused by a deluge.

Now, new high resolution bathymetry data of the entire channel in a new paper by marine geophysicist Jenny Collier of Imperial College and colleagues shows another key feature supporting the flood hypothesis has been found—underwater islands. There are 36 mid-channel islands that are formed from the bedrock, indicating these features were erosional, not depositional. “The islands are key here, as critics of our work suggest this is a “normal” river, in which case the islands would be depositional…we show they are erosional (made of solid rock),” Collier explained. They also exhibit a classic tear-drop shape that is indicative of islands formed during times of high water flow.

Evidence is mounting that this megaflood happened, but how? Even though the geomorphological data shows grooves and islands in the bedrock that could have been created by a flood, the timing of the event (or events) remains mysterious. Geologists hypothesize that the original megaflood occurred around 450,000 years ago, coincident with the time a vast ice sheet covering Britain joined up with one covering Scandinavia. Paleobiologist Tori Herridge of the Natural History Museum in London explains: “There is evidence for a huge pro-glacial lake stretching across what is now the North Sea basin: thus the lake was dammed by ice to north, and must have been dammed by something at the other end.” So what was damming the other end?

The extent of the ice sheet 450,000 years ago and the location of the megaflood. (Image provided by Jenny Collier)

The famous white chalk cliffs of Dover, England may have been the dam that was overtopped 450,000 years ago to cause the megaflood. Once the dammed glacial water overflowed the chalk dam, there was no stopping it. An absolutely catastrophic flood carved valleys and islands on the floor of the channel—and then evidence suggests it even happened again. There is additional evidence megaflood part II occurred at the end of the next most extreme glacial 160,000 years ago. Scientists posit that after this 2nd flood, Britain was officially an island except in the times of the absolute lowest sea levels (during the highest glacial periods).

The chalk that makes the white cliffs of Dover may have been the dam that was burst, causing the megaflood (Image via Wikimedia Commons CC BY-SA 2.0 by flickr user Fanny)

The biological implications of a megaflood are potentially immense. A vast release of water from behind a dam could not only impact drainage patterns and sediment transport, but it could alter ocean currents and cut off previously used migratory routes for plants and animals. The dating of these events is currently not precise enough to say whether these flooding episodes helped shape the unique composition of biota seen in the UK today. “Whether Britain was insular at all during the high sea-level periods of the other the intervening warms stages remains unclear, as of course the other intervening factor is the complex interplay between sea floor bathymetry and sea level rise,” Herridge said. So it still remains to be seen how exactly this interplay of physical factors prevented animal migration across Europe and Britain, if at all.

This complex story of the geological history of Britain and the English Channel is presented by Tori Herridge tonight, February 6th, at 8 PM GMT in the UK on the Channel 4 documentary Walking Through Time, where Collier and others also dive a chalk island and get up close and personal with the physical evidence of the megaflood.